Within the field of material handling, screw conveyors are employed in which is included a spiral thread (screw thread) manufactured of flat rolled steel placed on its edge, i.e. a screw with no mechanical center shaft, in which the screw is disposed in a casing and is rotated about its geometric center axis. In the description that follows, the expression "shaftless screw conveyor", "shaftless thread" and "shaftless screw" will often be employed to designate a screw conveyor with a spiral thread of the above design.
Prior art shaftless screw conveyors enjoy many advantages such as light weight, simple design, low production costs, generally high conveyance capacity, high operational reliability, small risk of damage etc. As a result, there are many incentives to extend the field of application of the shaftless screw conveyor so as to encompass materials which have hitherto not been conveyed or have been conveyed only with difficulty (low capacity, high driving power output, considerable wear) by means of shaftless screw conveyors. Examples of such materials are materials which readily become compacted and, consequently, form a compressed material layer in the conveyor between the screw and its casing. In the following description, the term "dense" materials will be employed for such materials, these consisting of, for instance, earth, compost, fine-grained sand, sludge with a high TS (total solids) content, fly ash, etc.
A shaftless screw conveyor is, in many physical applications journalled only at one end, the drive end, where it is also coupled to a drive unit. As a rule, the drive unit is also employed as a journal for the shaftless screw. Even if the shaftless screw possesses relatively high mechanical strength in its axial direction, it is so elastic in the radial direction that the screw, in an empty screw conveyor, rests against the lower inner surface of the U-shaped passage or casing after only one or a few helical turns. In certain physical applications, such screw conveyors are extremely long, lengths of 30-50 m being not uncommon. The elasticity in the radial direction is utilized to provide the possibility for the screw thread to deflect in the radial direction if pieces of material are jammed between the threads and the wall of the casing. Hereby, the risk will be eliminated that the shaftless screw is damaged by a lengthy deformation in the axial and/or radial direction.
In the design and manufacture of the shaftless screw conveyors, attempts are made to realize a spiral or helical thread which is flexible in the radial direction but is as stable as possible in the axial direction. Attempts are made in this art to achieve this goal by employing more and more robust -- and above all -- greater dimensions and by using steel materials of as "hard" properties as possible. Prior art common dimensions are 50.times.20, 60.times.25 and 80.times.25 mm. The employment of a "hard" steel is also advantageous from the point of view of resistance to wear. However, a restricting factor is that hard steels are difficult to shape. One skilled in the art will readily perceive that, in the production of a spiral thread from a flat-rolled steel, the hardness and dimensions of the steel and the radii of curvature of the spiral thread (internal and external) together make up the restricting parameters within which designers and manufacturers must adhere, for technical and economical reasons.
A screw thread of the type referred to above suffers from the drawback that, in certain physical applications -- and in particular when "dense" or heavy materials are to be conveyed -- the screw thread is lifted by the material which is to be displaced, since the rotating thread abuts against the material only by its own weight. In particular in physical applications in which the thread is exposed to large axially directed forces, for example since it is intended to be included in an apparatus with large displacement capacity and/or is adapted for the displacement of heavy materials, it is necessary, in the dimensioning of the screw thread, to impart such great mechanical strength to the thread that the thickness of the thread blade i.e. its extent substantially at right angles to the radial direction of the thread, entails that the abutment surface of the thread against the substrate will be large and, thereby, the abutment pressure of the thread (force per surface unit) against the substrate will be low. This leads in turn to the consequence that screw conveyors which include a shaftless spiral cannot be employed in certain physical applications, since the thread penetrates insufficiently down into the material which is to be displaced. The nature of the material itself influences the tendency for the screw to ride up and in particular when the material is fine-grained and heavy or when it is tacky and tends to adhere to the bottom of the conveyor passage or casing, this tendency to rise and ride up is amplified.
As already mentioned, it is of utmost importance that the spiral thread be designed so as to be configuratively stable in the axial direction. The pitch of the thread affects the axial configurative stability, since, for a shorter pitch the thread is more resiliently yieldable and compressible. In order to obtain an acceptable stability in the axial direction, a rule of thumb applies that the pitch of the thread be selected to be at least equal to the diameter of the thread. In order that transport capacity is not reduced, it is, however, necessary to employ, in inclined installations as short thread pitches as possible. As already stated, reduced pitch entails reduced mechanical configurative stability in the axial direction, i.e. the thread becomes more resiliently yieldable. As a result, in order to maintain configurative stability, the thread is made thicker which, of course, causes the abutment surface against the substrate to increases. As was mentioned above, a large abutment surface is disadvantageous, and is particularly sensitive in an inclined conveyor, since in such a construction, the slope alone entails that the abutment pressure to be reduced, and as a result, the lifting effect of the material on the screw thread is amplified. In addition, for optimum utilization of the transport capacity of the conveyor, a high filling degree is required, which further increases this lifting effect. The mutually conflicting design and construction requirements result in something of a serious dilemma.
In an inclining inclined conveyor there is often an undesirably high filling degree in its lower portion in that, for example, fine-grained material to some extent passes rearwardly over the blades of the spiral thread. Material hereby accumulates in the lower portion of the conveyor, which further intensifies the lifting effect on the screw thread in this lower portion. These difficulties have hitherto been overcome by welding guide vanes within the casing in positions above the spiral thread in order to prevent the thread from being lifted up by the material conveyed. Such guide vanes suffer, however, from the drawback that they increase friction, which leads to increased wear and to the need for greater driving power, which naturally results in increased costs for the plant. Moreover, the guide vanes reduce the possibilities of the thread to "snake" off in the radial direction if pieces of material become jammed between the thread and the wall of the passage or casing. As a result, the thread will be exposed to greater mechanical stresses and the risk of damage to the thread increases. Increasing the mechanical strength of the thread itself is no solution to these problems, since the lifting effect of the material being conveyed is thereby reinforced and the risk for jamming and seizure increases even more. Once again, mutually conflicting design and construction conditions may be identified.